Method of forming bearing ring resistant to season cracking

ABSTRACT

A method sets a retained austenite amount of an edge of a bearing ring of a bearing to between 3% or more and 20% or less. The method includes exposing the bearing ring to an environment having a temperature in the range of 850° C. to 930° C., an ammonia concentration, and a carbon concentration C·P in a range of 0.9 to 1.1%, for a period of not more than 5 hours. The ammonia concentration of the environment is in a range of 4 to 7 CFH. The temperature of the environment is then reduced to a temperature in a range of 800° C. to 830° C. for a second period of at least 30 minutes. The bearing ring is next quenched in oil and then heat treated in an environment set to a temperature of 160° C. to 200° C. and then air cooled.

BACKGROUND OF THE INVENTION

This invention relates to a rolling bearing unit having a rollingbearing attached to the outer periphery of a shaft body such as avehicle-use hub unit.

The vehicle-use hub unit, in general, has a hub wheel and a double rowrolling bearing. The rolling bearing is mounted on the outer peripheryof the shaft body of a hub wheel so as not to slip off the shaft body.

The shaft body of the hub wheel has on its free end side a cylindricalportion used for preventing the bearing from slipping off the shaftbody. The cylindrical portion is bent and deformed outward in a radialdirection using a caulking jig to be caulked onto an axial outer endface of an inner ring of the bearing. Thereby the cylindrical portionforms a caulking portion. The caulking portion prevents the bearing fromslipping off the hub wheel. At the same time, a pre-load is applied fromthe caulking portion to the inner ring of the bearing. Carbon steel isused as the material of the inner ring of the bearing.

In order to enhance the life of the bearing, the inner ring isheat-treated to increase the strength of a raceway surface thereof.

SUMMARY OF THE INVENTION

When the rolling bearing unit is left for the purpose of storage or thelike with the inner ring caulked onto the shaft body of the hub wheel, aphenomenon in which the inner ring is cracked and broken (hereinafter,referred to as season cracking) may be generated.

Accordingly, it is a main object of the invention to provide a rollingbearing unit preventing the generation of season cracking in the edge ofan inner ring of a rolling bearing.

Other objects, features and advantages of the invention will be apparentfrom the following description.

In one aspect of the invention, this is accomplished by providing arolling bearing unit comprising a shaft body, an inner ring and afastening member. The inner ring is mounted on an outer peripheralsurface of the shaft body. The inner ring is formed of carbon steel. Thesurface of the inner ring is heat treated and has a surface layer inwhich retained austenite exists. The fastening member is fastened to anaxial outer end face of the inner ring for applying a tensile stress inan outer peripheral direction of the inner ring. The inner ring has anedge on an outside in a radial direction thereof. The surface layer isremoved from the edge of the inner ring.

According to the rolling bearing unit of the invention, the surfacelayer, which causes season cracking, is removed from the edge of theinner ring. Therefore, the influence of a volume increase in thetransformation of retained austenite into martensite, is reduced. As aresult, the generation of the season cracking on the edge of the innerring due to an increase in a tensile strength is prevented.

Preferably, the surface layer is removed from edge of the inner ring bypolishing or turning. More preferably, an amount of the retainedaustenite existing in the edge of the inner ring is set between 3% ormore and 20% or less by removing the surface layer.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE.

These and other objects as well as advantages of the invention willbecome clear by the following description of preferred embodiments ofthe invention with reference to the accompanying drawings, wherein:

FIG. 1 is a sectional view showing a rolling bearing unit comprising ahub unit for a driving wheel according to an embodiment of theinvention;

FIG. 2 is a sectional view showing an inner ring of the rolling bearingunit according to the embodiment of the invention;

FIG. 3 is a sectional view showing an inner ring of a rolling bearingunit;

FIG. 4 is a view illustrating a surface layer generated on the surfaceof the inner ring of the rolling bearing unit; and

FIG. 5 is a sectional view showing an inner ring of a rolling bearingunit according to another embodiment of the invention.

In all these figures, like components are indicated by the samenumerals.

DETAILED DESCRIPTION

Referring now to the drawings, FIG. 1 illustrates a rolling bearing unitcomprising a vehicle-use hub unit for a driving wheel according to apreferred embodiment of the invention. A hub wheel 1 on which a wheel ismounted has a shaft body 2 as a rotatable shaft. A double row rollingbearing 3, which is specifically an angular ball bearing as one exampleof rolling bearings, is pressed and fitted onto the outer peripheralsurface of the shaft body 2 from the vehicle inner side.

The rolling bearing 3 has an inner ring 32. The inner ring 32 isattached to an annular groove 2 a in the outer peripheral surface of thevehicle inner side of the shaft body 2. The inner ring 32 has an innerring raceway 32 d, while the shaft body 2 has an inner ring raceway 2 bon the outer peripheral surface in the middle in an axial directionthereof. The inner ring raceway 2 b constitutes a pair of inner ringraceways in the axial direction together with the inner ring raceway 32d of the inner ring 32. Moreover, the shaft body 2 has an outward radialflange 2 c on the outer peripheral surface of the vehicle outer sidethereof.

A tire wheel and a brake disk, not shown, are attached to a side of thevehicle outer side of the flange 2 c. Furthermore, the rolling bearing 3has an outer ring 33. The outer ring 33 is fixed to a vehicle bodythrough a steering knuckle, not shown, and is provided with outer ringraceways 38 and 39 in two rows in the axial direction on the innerperipheral surface thereof.

The outer ring 33 is situated coaxially on the outside in the radialdirection with respect to the inner ring 32. The rolling bearing 3includes; balls 34 and 35 in two rows in the axial direction interposedbetween the inner ring raceways 32 d and 2 b and outer ring raceways 38and 39; cages 36 and 37 in two rows in the axial direction which holdthe balls 34 and 35; and seal rings 7 and 8 for sealing both ends in theaxial direction of the rolling bearing 3.

A shaft end of the vehicle inner side of the shaft body 2 is bent anddeformed outward in a radial direction and is caulked onto an outer endface 32 a on the vehicle inner side of the inner ring 32, therebyforming a caulking portion 4. The caulking portion 4 as a fasteningmember applies an appropriate pre-load to the balls 34 and 35 andprevents the rolling bearing 3 from slipping from the shaft body 2.

With reference to FIG. 2, carbon steel such as high carbon chromiumsteel (Japanese Industrial Standards SUJ2, bearing steel) or carbonsteels for machine structural use (Japanese Industrial Standards S55C)is used for the material of the inner ring 32.

In order to increase the surface strength of the raceway 32 d of theinner ring 32 to enhance the life of the bearing, the inner ring 32 isheat treated in a process for manufacturing the inner ring 32. The heattreatment is carried out in a slight carburizing atmosphere in order toprevent decarburization. Or the heat treatment is carried out in acarbonitriding atmosphere in which a small amount of NH₃ (ammonia gas)is added to an atmospheric gas.

As a result of an examination of season cracking, the following mattersare considered. With reference to FIGS. 4 and 5, description will begiven to the phenomenon in which the season cracking is generated.Numeral 32 denotes an inner ring and numeral 2 denotes a shaft body of ahub wheel. The end of the shaft body 2 is caulked onto an outer end face32 a on the end in the axial direction of the inner ring 32, therebyforming a caulking portion 4. A tensile stress in a circumferentialdirection is applied from the caulking portion 4 to a portion of theinner ring 32 in the vicinity of the caulking portion 4. Since the outerend face 32 a of the inner ring 32 is held by the caulking portion 4,the tensile stress thereof converges on an outer diameter side edge 32 bof the inner ring 32.

After the heat treatment of the inner ring, a surface layer L having anamount of retained austenite larger than other portions of the innerring 32 is formed on the surface of the inner ring 32 as shown in FIG.4.

An outer diameter side edge 32 b of the outer end face 32 a of the innerring 32 is constituted by the outer end face 32 a of the inner ring 32and an outer peripheral surface 32 c in a shoulder portion of the innerring. In the surface layer L of the outer diameter side edge 32 b, theretained austenite amount becomes larger than that of the other surfacelayer due to heating and carbon diffusion from the outer end face 32 aand the outer peripheral surface 32 c in the inner ring 32 during theheat treatment of the inner ring 32.

After the heat treatment, the retained austenite in the surface layer Lis transformed into martensite, so that the volume of the surface layerL is increased.

Together with the tensile stress applied in the outer diameter side edge32 b of the inner ring 32, the increase in the volume of the surfacelayer L by the martensite results in an increase in a tensile stress. Asa result the season cracking may be caused over the diameter side edge32 b of the inner ring 32.

As described above, it is considered that the retained austenite amountin the outer diameter side edge 32 b of the inner ring 32 causes thegeneration of the season cracking phenomenon.

Thus, in order to prevent the season cracking, this invention proposes arolling bearing unit capable of preventing the season crackingphenomenon by decreasing the retained austenite amount in the outerdiameter side edge 32 b of the inner ring 32.

Hereinafter, description will be given of a method of decreasing theretained austenite amount in the outer diameter side edge 32 b of theinner ring 32.

According to a first method, in view of a point that the retainedaustenite amount in the surface layer L gradually decreases from thesurface of the inner ring to the inside, the surface layer L in theouter diameter side edge 32 b of the inner ring 32 is removed until apredetermined depth by polishing or turning, thereby decreasing theretained austenite amount in the outer diameter side edge 32 b.

Accordingly, in the outer diameter side edge 32 b, the surface layer Lis removed such that the retained austenite amount becomes between 3% ormore and 20% or less in the range a in which at least tensile stressconverges, thereby reducing the percentage of generation of the seasoncracking in the inner ring 32.

A concrete range a is determined as follows.

The range a in a radius r, in which the surface layer is removed fromthe edge 32 b, satisfies the following expression:(φB−φA)/2≧r≧D

wherein φA represents an outer diameter of the caulking portion 4, φBrepresents an outer diameter of the outer peripheral surface 32 c in theshoulder portion of the inner ring 32, r represents a radius from theedge 32 b of the inner ring 32 in the vicinity of the caulking portion4, and D (mm) represents a numeric value which is almost equal to thethickness of the surface layer in which an amount of retained austenitewhich generates on the surface of the inner ring 32 is much more thanthat of other portions of the inner ring (for example, 20% or more). Thethickness of the surface layer is preferably 0.5 mm.

By removing the surface layer present within the range satisfying theexpression by the radius r from the edge 32 b of the inner ring 32,thus, retained austenite amount on the edge 32 b is between 3% or moreand 20% or less, preferably between 5% or more and 15% or less, morepreferably between 5% or more and 10% or less.

When the retained austenite amount was equal to or less than 20%, theseason cracking was not generated after the passage of the days. Whenthe retained austenite amount was 23%, the season cracking was generatedon a tenth day. When the retained austenite amount was 30%, the seasoncracking was generated on a seventh day.

According to the rolling bearing unit thus constituted, the retainedaustenite amount is set between 3% or more and 20% or less within therange of the radius r satisfying the expression from the edge 32 b ofthe inner ring 32 so that the influence of a volume dilatation can bereduced in the transformation of the retained austenite into martensite.As a result, it is possible to prevent the season cracking from beinggenerated on the edge 32 b of the inner ring 32 due to an increase in atensile stress caused by the caulking portion 4. Consequently, thereliability of the bearing can be enhanced.

The invention is not restricted to the embodiment in which the edge 32 bof the inner ring 32 is subjected to the polishing or the turning to setthe retained austenite amount in the edge 32 b to be between 3% or moreand 20% or less but the retained austenite amount in the edge 32 b maybe set between 3% or more and 20% or less by the optimization ofheat-treating conditions in the process for manufacturing the inner ring32.

Conventional heat-treating conditions are as follows. More specifically,the inner ring 32 is put in a heat treatment furnace. At this time, theinternal temperature of the heat treating furnace is set to 850° C. to930° C. The heat treating time for the inner ring 32 is set to fivehours. A carbon concentration C·P (Carbon·Potential) in the heattreatment furnace is set to 1.1 to 1.4%, and an ammonia concentration isset to 7 to 11 CFH. The CFH represents ft.sup.3/H (cubic foot per hour).Then, the internal temperature of the heat treatment furnace is droppedto 800° C. to 830° C. and the temperature dropping state is kept for 30minutes. After that the inner ring 32 is taken out of the heat treatmentfurnace and is then thrown into the oil kept at temperatures of 60° C.to 100° C. to be quenched (oil cooling). After the oil cooling, theinner ring 32 is tempered again for two hours in a heat treatmentfurnace having an internal temperature of 160° C. to 200° C.Subsequently, the inner ring 32 is taken out of the heat treatingfurnace and is air cooled.

In the invention, the conventional heat-treating conditions for theinner ring 32 are improved. More specifically, the inner ring 32 is putin the heat treatment furnace. At this time, the internal temperature ofthe heat treating furnace is set to 850° C. to 930° C. The heat treatingtime of the inner ring 32 is set to five hours. A carbon concentrationC·P (Carbon·Potential) in the heat treating furnace is set to 0.9 to1.1%, and an ammonia concentration is set to 4 to 7 CFH. Then, theinternal temperature of the heat treatment furnace is dropped to 800° C.to 830° C. The temperature dropping state is held for 30 minutes.Thereafter the inner ring 32 is then taken out of the heat treatmentfurnace and is thereafter put into oil kept at temperatures 60° C. to100° C. to be quenched (oil cooling). After the oil cooling, the innerring 32 is tempered again for two hours in a heat treatment furnacehaving an internal temperature of 160° C. to 200° C. Subsequently, theinner ring 32 is taken out of the heat treating furnace and is aircooled.

In the invention, thus, the carbon concentration and the Ammoniaconcentration in the heat-treating conditions are improved so that theretained austenite amount in the edge 32 b can be set between 3% or moreand 20% or less without polishing or turning the edge 32 b of the innerring 32.

The invention is not restricted to the inner ring 32 having thestructure described in the embodiment. For example, as shown in FIG. 5,a step 32 e having a diameter reduced is provided on the vehicle innerside end of the shoulder portion of the inner ring 32, and the step 32 eis set to be the installation space of a rotating speed sensor and thevehicle inner side end of the step 32 e is set to be the edge 32 b. Theretained austenite amount within the range of the radius r satisfyingthe expression may be set between 3% or more and 20% or less in the edge32 b.

In the invention, a fastening member is not restricted to the caulkingportion 4 in the embodiment but may be a nut screwed to the shaft body 2and fastened to an outer end face in the axial direction of the innerring 32. In brief, the shaft body 2 includes a fastening member which isfastened to the outer end face in the axial direction of the inner ring32 and applies a tensile stress in the outer peripheral direction of theinner ring 32.

The invention is not restricted to the hub unit comprising a combinationof the hub wheel and the angular ball bearing in the embodiment but thehub wheel or the like may be set to be the shaft body and a rollingbearing such as an angular ball bearing may be applied to variousrolling bearing units provided on the outer periphery of the shaft body.

1. A method of producing a bearing ring with a retained austeniteconcentration of 3% to 20% in an edge surface, comprising: forming abearing ring of carbon steel; and exposing the bearing ring to anenvironment having a temperature in the range of 850° C. to 930° C. anammonia concentration, and a carbon concentration C·P in a range of 0.9to less than 1.1%, for a period of not more than 5 hours.
 2. The methodof claim 1 further comprising setting the ammonia concentration of theenvironment in a range of 4 to 7 CFH.
 3. The method of claim 2 furthercomprising reducing the temperature of the environment to a temperaturein a range of 800° C. to 830° C. for a second period of at least 30minutes after the period of not more than 5 hours is expired.
 4. Themethod of claim 3 further comprising removing the bearing ring from theenvironment at the end of the second period and quenching the bearingring in oil at a temperature in a range of 60° C. to 100° C.
 5. Themethod of claim 4 further comprising tempering the bearing ring afterthe quenching in an environment set to a temperature of 160° C. to 200°C.
 6. The method of claim 5 further comprising air cooling the bearingring after the quenching.
 7. The method of claim 6 wherein the carbonsteel is high carbon chromium steel corresponding to Japanese IndustrialStandards SUJ2.
 8. The method of claim 6 wherein the carbon steel ismachine structural use steel corresponding to Japanese IndustrialStandards S55C.
 9. The method of claim 1 further comprising setting anammonia concentration of the environment is in the range of 4 to lessthan 7 CFH.
 10. The method of claim 9 further comprising reducing thetemperature of the environment to a temperature in a range of 800° C. to830° C. for a second period of at least 30 minutes after the period ofnot more than 5 hours is expired.
 11. The method of claim 10 furthercomprising removing the bearing ring from the environment at the end ofthe second period and quenching the bearing ring in oil at a temperaturein a range of 60° C. to 100° C.
 12. The method of claim 11 furthercomprising tempering the bearing ring after the quenching in anenvironment set to a temperature in a range of 160° C. to 200° C. 13.The method of claim 12 further comprising air cooling the bearing ringafter the quenching.
 14. The method of claim 13 wherein the carbon steelis high carbon chromium steel corresponding to Japanese IndustrialStandards SUJ2.
 15. The method of claim 13 wherein the carbon steel ismachine structural use steel corresponding to Japanese IndustrialStandards S55C.
 16. A method of producing a bearing ring with a retainedaustenite concentration of 3% to 20% in an edge surface, comprising:forming a bearing ring of carbon steel; and exposing the bearing ring toan environment having a temperature in the range of 850° C. to 930° C.,an ammonia concentration of the environment in the range of 4 to lessthan 7 CFH, and a carbon concentration for a period of not more than 5hours.
 17. The method of claim 16 further comprising setting the carbonconcentration C·P in a range of 0.9 to less than 1.1%,
 18. The method ofclaim 17 further comprising reducing the temperature of the environmentto a temperature in a range of 800° C. to 830° C. for a second period ofat least 30 minutes after the period of not more than 5 hours isexpired.
 19. The method of claim 18 further comprising removing thebearing ring from the environment at the end of the second period andquenching the bearing ring in oil at a temperature in a range of 60° C.to 100° C.
 20. The method of claim 19 further comprising tempering thebearing ring after the quenching in an environment set to a temperaturein a range of 160° C. to 200° C.
 21. The method of claim 20 furthercomprising air cooling the bearing ring after the quenching.
 22. Themethod of claim 21 wherein the carbon steel is high carbon chromiumsteel corresponding to Japanese Industrial Standards SUJ2.
 23. Themethod of claim 21 wherein the carbon steel is machine structural usesteel corresponding to Japanese Industrial Standards S55C.